AC synchronous motors and brushless DC motors are controlled through commutation of solid state switching devices connected to their stator windings. These motors can be of the permanent magnet (PM) type in which permanent magnets are used on the rotor instead of rotor windings. Permanent magnet synchronous motors (PMSM) are widely used in motion controls, electric vehicles, and industrial turbo generators (ITG).
For demanding servo applications, having a well-tuned torque loop in an electric motor drive that works optimally under a variety of operating points is highly desirable. In a multi-axis servo application, suboptimal torque loop performance on any axis at any time can adversely affect servo performance in terms of adequately achieving the required velocity response and steady-state behavior.
Tuning the torque loop in an electric motor drive can be a labor intensive exercise that involves expensive equipment and often tuning techniques involve tuning for a particular operating point. However, due to the nonlinear dynamics of the cross-coupling terms in the voltage machine equations, optimal tuning at one operating point can be very suboptimal for a different operating point. For example, the majority of control schemes in industry utilize conventional PI control. These controllers typically have only one set of gains, and are optimized for a single operating point. Operation at any other point tends to be sub-optimal. Since these controllers are based on non-linear three-phase motor equations, their robustness and performance declines with changes in current, speed and inductance. Thus with conventional techniques one is left with a situation where performance and adequate stability margins are not guaranteed across the entire torque versus speed curve for a particular motor. Therefore, there is a need for a system that tunes high speed motors with wide operating ranges.
Servomotor applications typically require that the servomotor be capable of operating at various speeds. Motors can be driven by placing a pulse width modulated (PWM) inverter and PWM controller between the servomotor and the voltage source. Ideally, one would like to choose a PWM switching frequency as high as possible. However, certain parameters put a constraint on just how high the switching frequency can be obtained. There are a number of factors that influence the selection of the PWM switching frequency in three-phase inverters. Among these factors are:
(1) Switching losses;
(2) Inductance of the electric machine;
(3) Control algorithm selection;
(4) Electromagnetic interference (EMI) emissions; and
(5) Electric machine harmonics.
One of the constraints in choosing the frequency is the processing capability of the computing element of the motor drive. The processor quite simply has a finite limit in how fast it can generate a control output. Another constraint is the switching losses. It is well known that switching losses are linearly proportional to the switching frequency. Depending on the switching elements selected in the inverter design, this can have significant impact on inverter efficiency which affects the design of the heat sink and vehicle cooling system sizing. Selection of an appropriate control algorithm with an appropriate PWM switching frequency can add important benefits toward minimizing this impact. There is a sizeable body of research in the technical community that has been focused on developing control algorithms that can lower switching losses and analyzing and comparing these control schemes to determine their impact on switching losses.
To solve the problem, one would like to set up a set of circumstances where the time spent on tuning the torque loop can be minimized while at the same time the techniques guarantee stability margins and performance requirements that hold across the entire torque versus speed curve for the motor under test. This is a complex problem that requires the following multi-layered strategy in order to solve. Thus there is a need to:
(1) Develop compensation for parameters that can change as a function of some known variable. It is known that motor torque constant, for example, changes as a function of current. By compensating for varying parameters in a well-behaved manner, they can be treated as constants for practical purposes and this simplifies the problem to a certain degree to set the stage for the next layer in the strategy.
(2) Develop a means whereby the control technique can decompose the operating regions into manageable partitions and the control system automatically adjusts its parameters according to which region it is operating in.
(3) Develop a set of performance enhancement techniques that can be implemented as needed to further improve performance. These techniques would further enhance torque loop response time, minimize current ripple and improve torque output for a given motor size.
(4) Develop a tool in firmware that can verify the performance enhancements and stability margins without the need for expensive equipment.